Lay-in lugs have a "C" shape to permit laying in a wire. Such lugs may be used in a wide variety of applications and are especially well suited for use as a grounding device for electrical equipment such as bushings, or the like, and will be particularly described in this connection. Lay-in lugs for electrical fittings are disclosed in U.S. Pat. Nos. 3,365,693, issued Jan. 23, 1968, to Frank L. Browne; and 3,706,959, issued Dec. 19, 1972, to Alexander R. Norden; both of which are assigned to the same assignee as the present invention. Grounding lugs are provided to facilitate the grounding of exposed metallic surfaces such as a conduit and/or other electrical fittings. The grounding connection is frequently made through a bushing which may be threaded onto the end of a conduit as at a service box where the conduit passes through a wall of the box. Grounding lugs of this type are generally adapted to be secured by one or more screws to the periphery of the conduit bushing, and are adapted to receive the grounding wire, preferably without requiring the wire to be cut. The lay-in lug is convenient to use but, when tightened, comprises a highly stressed structure and must, therefore, be fabricated of suitable materials having a high yield strength. If a high yield strength material is not used, adequate pressure could not be applied to the laid-in wire. Typically, a clamping screw is provided for securing the wire within the lug. Under ground fault conditions, a very large ground current may pass through the grounding wire and the lug; and therefore it is important that the lug provide a suitable low-resistance connection. Under fault conditions the ground current may rise to thousands of amperes. Unless good conductivity is provided between the grounding wire and the bushing, the large current may cause sufficient heat to melt or even vaporize the lug. This may result in loss of the grounding connection, circuit failure, danger to personnel and equipment damage or fire. Accordingly, efforts have been made to improve the conductivity between the grounding wire and the bushing, or the like.
Prior art devices have, for the most part, attempted to provide the desired low resistance by providing more massive lugs and/or larger areas of contact together with large supporting screws. These devices must be made of a high-strength material so that the connector will not yield appreciably when a wire or cable is securely tightened by the wire-holding screw. Unfortunately, materials with high-yield strength have poor conductivity when compared with materials such as copper, aluminum or their alloys. Therefore, connectors made from high-strength materials will be subject to significant internal heating in response to the presence of a large fault current. The large currents, frequently encountered in the field under ground fault conditions, severely limit the utility of grounding lugs that have a high internal resistance, for two reasons. One is that the connector will become hot while passing a large current because of the internal resistance of the connector material and thereby cause it to melt and destroy the connector, which results in a discontinuity of the electrical grounding circuit. Another limitation is the result of the fault current being passed through the connector's mounting screw, causing the screw to heat and melt and thereby break the grounding circuit. This is a direct result of current being shunted through the screw, since the alternate current paths through the connector's body have a high resistance relative to that of the screw.